The invention relates to temperature indicating compositions which display temperature dependent color changes. The invention encompasses temperature indicating compositions, materials, devices and methods of their use. The temperature indicating compositions of the invention are clays that can be used to detect temperature and temperature changes at sub-ambient and particularly sub-zero xc2x0 C. temperatures.
A wide variety of materials and methods are available for detecting temperature and temperature changes. Perhaps the most common example of a temperature indicating device is the mercury bulb thermometer. Bulb thermometers rely on the simple principle that the volume of a liquid expands upon heating and diminishes when cooled. Mercury bulb thermometers have become undesirable since mercury is highly toxic. In fact, many communities and areas in the United States are actively banning or limiting the use of mercury and mercury thermometers. Alcohol thermometers are being used in place of mercury thermometers and have a lower usable temperature range, generally from about xe2x88x9220xc2x0 C. to about 200xc2x0 C.
Electronic methods for monitoring temperature are also available. The electronic methods commonly rely on the changes in electrical resistance of materials as temperature changes. Changes in resistance are measured electronically to yield an output signal which can be correlated to temperature. Typically, electronic methods for measuring temperature are more expensive than bulb thermometers and other temperature indicating devices since electronic devices require electronic circuitry and/or computer interface.
Other types of temperature sensing devices can be used in applications for controlling temperature. The bimetallic strip thermometer is used in ovens and refrigerators alike to sense and control temperature. On a general level the bimetallic strip thermometer has two strips of metals or alloys fixed to one another which expand at different rates as temperature changes. The bimetallic strip is also in direct contact with a wire. As temperature changes, the bimetallic strip curves due to the different rates of thermal expansion of the metal strips and comes in contact with an electrode which allows current to flow through the bimetallic strip. The temperature is simply controlled by adjusting the size of the gap between the bimetallic strip and the electrode. The flow of current is linked to a mechanism for controlling temperature, i.e., a heat pump or a heater.
Another common device used for measuring temperature is called a thermocouple. A thermocouple consists of two wires made of different metals, or alloys, joined at each end. Typically, one junction is placed where the temperature measurement is being made, and the other is kept at a constant lower temperature. The Seebeck effect produces an electromotive force that is approximately proportional to the difference between the temperatures of the two junctions.
Type T thermocouples made of copper and constantan are commonly used to measure temperatures below 0xc2x0 C. and typically have a range of xe2x88x92200xc2x0 C. to about 350xc2x0 C. Thermocouples need to be calibrated before use or have the temperatures read from standard tables and are expensive since they require circuitry and/or computer interface.
Although materials and methods for indicating temperature have existed for centuries, there is a continuing need for compositions, materials, and methods for indicating temperature which are inexpensive and amenable to facile use at sub-ambient and sub-zero xc2x0 C. temperatures.
The invention provides materials and methods for detecting changes in temperature by detecting a change in color of a clay material. The clay materials of this invention and methods employing them are useful for detecting temperature changes at sub-ambient temperatures, and particularly at temperatures below about 0xc2x0 C. The clay material of this invention or a temperature-sensitive composition comprising the clay material of this invention exhibits changes in color over a range of sub-ambient temperatures. Temperature of a selected environment can be monitored by placing a temperature-sensitive composition comprising the clay material of this invention in contact with that environment and monitoring the color of the composition. A change in temperature is detected by detecting the color change visually or instrumentally.
The invention specifically relates to a clay material that changes color at temperatures below 0xc2x0 C. Clay material that changes color at temperatures between about 0xc2x0 C. and xe2x88x92270xc2x0 C. is specifically disclosed. A particular disclosed clay material exhibits a dramatic color change from dark brown to bright green over this temperature range. The clay materials of this invention can be used to qualitatively detect an increase in temperature (increasing from below about xe2x88x92270xc2x0 C. through about 0xc2x0 C.) or to qualitatively detect a decrease in temperature (decreasing from above about 0xc2x0 C. through about xe2x88x92270xc2x0 C.). In a specific embodiment, the clay material is a dark brown colored (at ambient temperatures and depending on its water content), high-iron nontronite that changes color on cooling below about 0xc2x0 C. through about xe2x88x92270xc2x0 C. from greenish-brown, through yellow-green to bright green.
The invention includes temperature-sensitive compositions comprising the clay of this invention present in an amount ranging broadly from about 0.1% to about 99.9% by weight. Preferred compositions are listed in Table 1.
The invention also provides temperature indicators and temperature probes comprising a temperature-sensitive composition of this invention. These indicators or probes can take a variety of forms: powders, films, surface coatings, wafers, tubes, inks, paints, waxes, gels, and shaped articles (e.g., plates, vessels, containers, caps, etc.) with clay material admixed, suspended, impregnated therein or applied as a surface coating thereto.
The invention provides methods of detecting temperature and temperature changes. According to one embodiment of the invention, a method for detecting temperature comprises the steps of (a) contacting a temperature indicating material or device comprising a temperature indicating clay with an environment, and (b) determining the color of the temperature indicating material or device; whereby the temperature of the environment is correlated to the color of the material or device. According to another embodiment, the invention provides a method of detecting temperature change comprising the steps of (a) contacting a temperature indicating material or device comprising a temperature indicating clay with an environment, (b) determining the color of the temperature indicating material or device when it is in thermal equilibrium with the environment, and (c) monitoring the color of the temperature indicating material or device for color change; whereby a temperature change is detected when the color of the temperature indicating material or device changes.